A number of airports in the United States require airplanes to follow noise abatement procedures when taking off and climbing out. Typically, these procedures require a steep climb to a designated altitude. During the climb the pilot flies at a recommended airspeed which is a function of aircraft weight and flap setting. This steep climb is followed at the designed altitude by a sharp reduction in thrust and climb angle to achieve a minimum required climb gradient (and recommended airspeed) until the airplane is out of the noise abatement area. In this manner the airplane traverses the noise abatement area at a maximum altitude and minimum thrust level so that the perceived noise on the ground is minimized.
Previously, during the time the aircraft was transitioning from the steep climb angle/high thrust mode to the relatively shallow climb angle/low thrust mode, the pilot simply estimated the throttle position necessary to maintain the required airspeed and climb gradient. Typically, this procedure involved manually readjusting the throttle position and aircraft pitch attitude several times by trial-and-error before the correct throttle position and pitch attitude was finally reached.
Due to the time and effort involved in determining the correct throttle position, it is preferable that the throttle be positioned automatically during this transition to avoid the aforementioned trial and error procedure and to allow the pilot to direct his attention to other matters.
Conventionally, a number of autothrottle devices have been disclosed. For example, U.S. Pat. No. 4,019,702 by Annin (assigned to the assignee of the present application) discloses a pilot guidance system that displays pitch and speed commands during takeoff climb for engine noise abatement purposes.
Furthermore, U.S. Pat. No. 4,662,171 by Jackson et al (assigned to the assignee of the present application), the contents of which is incorporated by reference herein in its entirety, discloses an autothrottle control system for automatically increasing the thrust of a working engine(s) of an aircraft if power is lost on a different engine while the aircraft is operating at reduced thrust during a takeoff noise abatement maneuver.
These conventional devices, however, do not automatically position the throttle to maintain a selected climb gradient when the pilot is making a reduction in pitch attitude.